The use of microbial inoculants (such as Rhizobium) has long been known to increase crop yields. The utility of this approach has been limited by the availability of compositions capable of both affixing the microbial cells to the seed and maintaining the viability of the microbial inoculant. Burton, J. C. (U.S. Pat. No. 2,995,867) discloses the possibility of using water soluble gums such as methylcellulose as a seed coating. Through the use of such coatings, it is possible to keep microbial cells alive and effective for over two weeks.
The success of this work spurred others to attempt to identify alternate compositions which might be capable of maintaining bacterial viability for longer periods of time. A method was disclosed by Hamrin, B.S.A. (U.S. Pat. No. 3,688,437), which involved the use of seed coatings comprising a gelatin-calcium alginate foam. Dommergues, Y. R., et al. (U.S. Pat. No. 4,155,737) disclosed the possibility of embedding the microbial inoculant in a polyacrylamide seed coating.
The possibility of stabilizing bacterial cells in a microbial inoculant through the use of a polysaccharide matrix was shown by Mugnier, J., et al. (French Patent 2,469,861). This patent discloses the ability to stabilize a suspension of bacterial cells by embedding them in an alginate gel. The gel disclosed in this patent is formed by the addition of calcium chloride to a solution containing a suspension of bacterial cells and sodium alginate. The addition of calcium chloride results in the precipitation and crosslinking of the previously soluble sodium alginate mixture.
A similar alginate composition is disclosed by Jung, G. (European Patent 17,565 and U.S. Pat. No. 4,434,231). These patents disclose the possibility of crosslinking or precipitating solutions of sodium alginate that contain suspensions of bacterial cells. The Jung patents disclose the use of calcium sulfate to convert a sodium alginate suspension into a gel. This crosslinking reaction can be delayed through the addition of sodium phosphate to the suspension. Bacterial cells treated in this manner remained viable for greater than 60 days. These patents further disclose the use of such embedded microorganisms as microbial inoculants in agriculture. Although the ease of formation of crosslinked alginate gels commended their use as an embedding matrix for a microbial inoculant, such matrices were found to be inferior to embedding matrices of xanthan gums in that embedding matrices composed of xanthan gums were found to release microorganisms into the soil more readily than matrices composed of alginates. (Jung, G., et al., Plant and Soil, 65:219-231 (1982)).
Jung, et al. (European patent no. 83267) disclose compositions containing microbial cells and polymeric gels which may be dried and used to inoculate plant seeds. The patent further discloses the use of a crosslinked alginate suspension of microbial cells as a microbial inoculant.
One concern reflected in the prior art is that suitable embedding agents should provide an environment strong enough to resist external abrasion and adverse forces yet be pliable enough to allow release of internal components and permit breakdown of the matrix at the appropriate time. Two different methods have been disclosed in order to accomplish this goal. Redenbaugh, M. K., et al. (P.C.T. Publication W085/02972) disclosed the use of crosslinked alginate-bacterial cell suspensions which are surrounded by an external membrane as a microbial inoculant. Fravel, D. R., et al. (Phytopatholoqy, 75:774-777 (1985)) disclosed the use of crosslinked alginate-bacterial cell suspensions which contain clays or other materials, added as bulking agents. The process disclosed by Fravel, et al produces solid pellets of relatively uniform size.
Thus, the prior art teaches the value of pre-inoculating seeds with microbial suspensions in order to enhance their germination and growth. A wide variety of different embedding matrices have been disclosed in the prior art. Especially significant are matrix compositions which employ crosslinked or precipitated alginates or very thick, gum-like compositions. Such compositions have been successful in sustaining bacterial cell viability for extended periods of time, while simultaneously possessing the desired properties of rigidity and support. The preparation of gels, however, requires additional steps and both gels and gums involve handling difficulties.